Dark and sunlight-driven dye degradation over a TiO2–dibenzoylmethane hybrid xerogel

The pursuit of sustainable and inexpensive systems for the degradation of organic aquatic pollutants working under sunlight or even without irradiation is still open. Herein, an amorphous titanium oxide modified with dibenzoylmethane, an aromatic diketone, is proposed as a solar photocatalyst showing oxidative activity also in the dark. Ti(IV)-dibenzoylmethanate (dbm) complexes form during the sol-gel synthesis and are dispersed throughout the oxide gel structure. This ligand-to-metal charge transfer complex plays a dual role: it enables visible light absorption, narrowing the apparent band gap, and provides localized surface charge, promoting the spontaneous formation and stabilization of superoxide radical ions by reduction of O2. The activity of the TiO2-dbm hybrid was assessed in the removal of methylene blue from aqueous solution. The results demonstrated a rather stable photocatalytic activity under natural sunlight, exceeding 80% dye removal after 1 h in four subsequent tests, a value that was mostly recovered after a mild regeneration. Considerable efficiencies were also achieved in the dark, owing to the immobilized superoxide radicals acting as initiators of degradation pathways. Finally, the influence of the operating conditions (pH, illumination, mixing method, water contact) on the efficiency and durability of the performances was evaluated to survey the long-term utilization in water decontamination. The behaviour of this hybrid material resembles a “photocatalytic memory” effect and may widen the perspectives for the development of heterogeneous advanced oxidation processes.